A new "morning after" pill more than halves the risk of pregnancy compared with the 72-hour pill available, researchers have said.
The new pill, which can prevent pregnancy for up to five days after unprotected sex, was licensed in Europe last May but is not yet available over the counter in the UK, and costs three times more than the traditional emergency pill, levonorgestrel.
Ulipristal acetate (UA) could be made available via pharmacies and nurses but needs more safety data to be made more accessible.
The trial found dizziness was the mos serious side effect, said doctors who published their study in TheLancet. They combined their own findings with data from another study.
Women seeking emergency contraception within five days of unprotected sex were randomly given one of the two pills at clinics in the UK, Republic of Ireland and the US. UA cut the risk of pregnancy by nearly two-thirds compared with levonorgestrel which the researchers said was safe, being used by millions of women without prescription being needed.
Pro-choice campaigners hailed UA as an "exciting" step forward.